36 research outputs found
Microstructures and sclerochronology of exquisitely preserved Lower Jurassic lithiotid bivalves: Paleobiological and paleoclimatic significance
No full textLithiotids are enigmatic, large-sized bivalves that formed an important biotic component of tropical shallow marine environments during the Early Jurassic. Lithiotis problematica and Cochlearites loppianus are the most peculiar lithiotids, characterized by stick-like shells of predominantly aragonite which is generally calcitized or replaced by sparry calcite. Uniquely preserved specimens of these two species from the upper Sinemurian-Pliensbachian (Lower Jurassic) Rotzo Formation (Trento Platform, northern Italy), containing large parts of pristine shell (based on SEM, cathodoluminescence, and μXRF analysis), were selected for a sclerochronological and sclerochemical study, allowing to describe in detail the lithiotid microstructures, to decipher seasonal patterns and to investigate their functional and paleoenvironmental significance. We show that the outer shell layer of lithiotids, rarely preserved, consists of a calcitic simple prismatic microstructure with an asymmetrical thickness between the two valves, whereas the inner layer is aragonitic and is mainly composed of a fibrous irregular spherulitic prismatic fabric, which allowed a very fast shell growth. The latter microstructure is currently unknown in other mollusc shells. We recognized diurnal, fortnightly and annual growth increments, documenting a maximum annual growth of about 25 mm/yr in ventral direction. Stable isotopes show a clear annual periodicity suggesting seasonal changes in the paleoenvironment, which also affected the shell microstructures. During the warm season, first-order prisms are very elongated and show a massive structure without growth breaks, whereas during the cold season prisms are short and with growth cessations. Our results highlight the unique adaptation of lithiotid bivalves that allowed them to dominate the tropical shelf seas during the Early Jurassic
Micro-X-ray fluorescence elemental data of the shell of A. benedeni benedeni specimens SG-125, SG-126, and SG-127
No full textThis dataset (XRF_Benedeni_benedeni_SG-125_126_127_crosslines_cal.csv) contains micro-X-ray fluorescence elemental data of the shell of A. benedeni benedeni specimens SG-125, SG-126, and SG-127. The bivalve shells were collected in 2013 from a shell bed at the top of the Oorderen Member of the Lillo Formation of the Pliocene in Belgium. The collection site was a construction-related temporary outcrop at the Deurganck Dock Lock (now Kieldrecht Lock) in the city of Antwerp, located at 51°16′44″N 4°14′52″E. These measurements were carried out in order to screen for diagenetic alteration, which results in enrichment of certain elements (e.g., Fe and Mn). The specimens were measured in spring 2021 at the Analytical, Environmental, and Geochemistry Research Group (AMGC) of the Vrije Universiteit Brussel, Belgium), on a Bruker M4 Tornado µXRF scanner. This instrument is equipped with a 30 W Rh anode metal-ceramic X-ray tube operated at 50 kV and 600 µA, and polycapillary lens focussing. The measurement was carried out following the methods described in de Winter and Claeys (2017) and Kaskes et al (2021), and consisted of multiple line scans oriented perpendicular to the growth direction. Also provided is an image file (XRF_Benedeni_benedeni_crosslines_locations.png) which shows the location of these line scans on the shell cross sections
Petrophysical data for 29 samples from the Chicxulub impact crater.
No full textNote: ɸ-porosity, ρb-bulk density, ρg-grain density, k-permeability, F-formation factor, m-cementation exponent, τ2-tortuosity, Cs-surface conductivity, Vp-acoustic velocity of compressional waves. Uncertainty for porosity, density, permeability, velocity and conductivity is 5%. Uncertainty for formation factor, cementation exponent and tortuosity is 8%). Lith 1 and Unit 1 after Morgan et al. (2017), Unit 2 after de Graaf et al. (2021, UIM-upper impact melt rock unit, LIMB-lower impact melt rock-bearing unit)) and Kaskes et al. (2021).
Morgan, J. V., Gulick, S. P. S., Bralower, T. J., Chenot, E., Christeson, G. L., Claeys, P., et al. (2016). The formation of peak rings in large impact craters. Science, 354(6314), 878–882. https://doi.org/10.1126/science.aah6561
de Graaff, S. J., Kaskes, P., Déhais, T., Goderis, S., Vinciane, D., Ross, C. H., et al. (2021). New insights into the formation and emplacement of impact melt rocks within the Chicxulub impact structure, following the 2016 IODP-ICDP Expedition 364. Geological Society of America Bulletin. https://doi.org/doi: https://doi.org/10.1130/B35795.1
Kaskes, P., de Graaff, S. J., Feignon, J. G., Déhais, T., Goderis, S., Ferrière, L., et al. (2021). Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization. Geological Society of America Bulletin. https://doi.org/https://doi.org/10.1130/B36020.1This table synthetizes data relevant to the petrophysics of the lithologies recovered from Chicxulub impact crater peak ring. Cores were extracted in 2016 during IODP-ICDP Expedition 364 Chicxulub K-Pg Impact Crater (Hole M0077A), and the 29 samples presented in the table were analysed in 2017-2018 at the University of Montpellier.
The samples are re-used shipboard discrete P-wave samples, they were selected to be representative of the diversity of lithologies encountered in Hole M0077A, dominated by shocked granites (19 samples) capped and cut by suevites (6 samples), impact melts (3 samples) and cataclasites (1 sample). Petrophysical measurements were made in order to bring new insight on the core and its heterogeneity, with implications for large scale processes such as hydrothermal systems.
Porosity (ɸ in percent) and density (⍴b, ⍴g, bulk and grain density in g/cm3) were calculated using the triple weight method. The permeability k (in milliDarcies, where 1 mD = 10-15 m2) was measured on dry samples using steady state conditions at an effective pressure of 10 and 40 MPa. Formation factor (F, unitless) and surface conductivity (Cs in mS/m) were determined using Waxman and Smits (1968) equation after saturating the samples with fluids of known conductivities. The cementation exponent (m, unitless) and tortuosity (τ2, unitless) were calculated knowing the formation factor and the porosity (Archie, 1942; Walsh and Brace, 1984). Acoustic velocities (Vp, in km/s) were measured on dry and saturated samples at a 500 kHz ultrasonic frequency using coupled piezoelectric, a pulse generator and an oscilloscope.
References:
Archie, G. E. (1942). The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics. Transactions of the AIME, 146(01), 54–62. https://doi.org/10.2118/942054-G
Walsh, J. B., & Brace, W. F. (1984). The effect of pressure on porosity and the transport properties of rock. Journal of Geophysical Research, 89(B11), 9425. https://doi.org/10.1029/JB089iB11p09425
Waxman, M. H., & Smits, L. J. M. (1968). Electrical Conductivities in Oil-Bearing Shaly Sands. Society of Petroleum Engineers Journal, 8(02), 107–122. https://doi.org/10.2118/1863-
Gold mineralization in the Mesoproterozoic Karagwe-Ankole belt (Byumba, Rwanda): new insights from petrography and trace element mapping
No full textsponsorship: We would like to thank Desert Gold Ventures Inc., Francis Gatare and Alain Ntenge from the Rwanda Mines, Petroleum and Gas Board for their collaboration and contribution. This research is financially supported by Research Grant C14/17/056 of the KU Leuven Research Fund. P. Claeys thanks Research Foundation Flanders Hercules Program for financing the Micro-XRF instrument (KU Leuven Research Fund|C14/17/056, Research Foundation Flanders Hercules Program)status: Publishe
Elemental concentration of Late Cretaceous flint samples from the Hallembaye quarry, Belgium
No full textElemental concentration of Late Cretaceous chalk samples from the Hallembaye quarry, Belgium
No full textElectron backscatter diffraction maps of thin sections of A. benedeni benedeni specimens SG-125, SG-126, and SG-127
No full textThis dataset (.crc and .cpr files) contain electron backscatter diffraction maps of thin sections of A. benedeni benedeni specimens SG-125, SG-126, and SG-127. Also included are .BMP files for each map. The bivalve shells were collected in 2013 from a shell bed at the top of the Oorderen Member of the Lillo Formation of the Pliocene in Belgium. The collection site was a construction-related temporary outcrop at the Deurganck Dock Lock (now Kieldrecht Lock) in the city of Antwerp, located at 51°16′44″N 4°14′52″E. These measurements were carried out in order to determine the microstructures of the shells of A. benedeni benedeni, as well as to screen for minor diagenetic alterations. The specimens were measured in spring 2021 at Department of Earth Sciences of Utrecht University, the Netherlands, on an Oxford Instruments Symmetry EBSD detector attached to a Zeiss Gemini 450 SEM. Thin sections of the samples were mechanically polished with 0.3 µm aluminium oxide suspension and finished with chemical Syton® polish. The thin sections were covered with a thin (several nm) carbon coating to keep charge build-up during the measurements at a minimum
Oxygen and carbon isotope data of the shells of A. benedeni benedeni specimens SG-126 and SG-127
No full textThis dataset contains oxygen and carbon isotope data of the shells of A. benedeni benedeni specimens SG-126 and SG-127, as well as two in-house standards (Naxos, Kiel-carbonate). These measurements were carried out in order to establish a sclerochronological record of year-by-year shell growth. The bivalve shells were collected in 2013 from a shell bed at the top of the Oorderen Member of the Lillo Formation of the Pliocene in Belgium. The collection site was a construction-related temporary outcrop at the Deurganck Dock Lock (now Kieldrecht Lock) in the city of Antwerp, located at 51°16′44″N 4°14′52″E. Oxygen and carbon stable isotopes were measured in spring 2021 at the department of Earth Sciences at Utrecht University, the Netherlands, on a Thermo Scientific GasBench II gas preparation system coupled to a Thermo Scientific MAT 253 isotope ratio mass spectrometer and using a LIDI workflow. The dataset includes raw data and corrected values, and final values are reported relative to the Vienna Pee-Dee Belemnite (VPDB) standard
Elemental concentration of Late Cretaceous chalk and flint samples from the Hallembaye quarry, Belgium
No full textThis dataset contains the elemental concentration (expressed in weight per cent, either as element or as oxide), measured from 2019 to 2021 using a Bruker M4 Tornado micro-X-ray fluorescence (AMGC, VUB, Belgium), of Late Cretaceous chalk and flint samples that were sampled in 2019 at the Hallembaye quarry (Belgium). The data was collected to (i) investigate the imprint of astronomical forcing on the geochemical composition of chalk and to (ii) assess the geochemical composition of flint samples
NON-TRADITIONAL STABLE ISOTOPE VARIATIONS IN THE IMPACTITES OF THE ROCHECHOUART IMPACT STRUCTURE: TRACING IMPACT VOLATILIZATION, MELTING, MIXING, AND HYDROTHERMAL OVERPRINTING
No full textIntroduction: The Rochechouart structure is a deeply eroded impact crater formed ~ 207 Myr ago and that no longer displays any impact-related topography [1]. Its surface is currently at the level of the crater floor, with a di-ameter of about 20-25 km, based on morphological, geophysical, and structural reconstructions [2,3]. Despite its high erosion degree, the Rochechouart impact structure displays a preserved suite of impactites (impactoclastites, suevites, impact melt rocks, breccias, granite and gneiss from basement), with a large proportion of melted material. These lithologies were sampled during the 2017 drilling campaign, held by the CIRIR in 2017 [4] (funded by the Réserve Naturelle Nationale de Rochechouart-Chassenon). This campaign resulted in 18 drill holes (with a cumula-tive length of ~ 540 m) located at 8 sites along two 10-km radial transects across the center of the structure [5].
Samples and methods: Nineteen samples from 6 drilling sites from across the Rochechouart impact structure have been selected for this study. The selected samples are from the cores SC1, SC2, SC3, SC7, SC11, SC15, SC16, and SC17. Three of them are from the basement (1 granite and 2 gneiss), 8 from impact melt rocks, 3 from suevites, 3 from impact breccias, and 2 from impactoclastites intervals. The nineteen samples selected for this study have been selected to complement the first petrographic and geochemical studies held. The aim is to trace a meteoritic component thanks to germanium [Ge] isotopic variations (CRPG-CRNS Nancy) and highly/moderately siderophile elements concentrations among the different lithologies selected. Volatilization may also be traced by Ge isotopic variations. Iron, zinc, and copper isotope systematics are also complementing the Ge isotope data to study other syn- and post-impact processes (melting, mixing of target rocks, hydrothermal alteration).
Results: The Ge isotope results for the 19 selected impactites and target lithologies are the first obtained within any impact structure. They display a strong varia-tion in d74/70Ge (from ~ 0.1 to 1‰). The observed Ge isotope compositions, in combination with their Ge con-centrations, put forward 2 groups among the impactite samples, distinct from the basement samples. One group (mainly impactoclastites and suevites) exhibits similar Ge concentrations as the basement samples but at higher d74/70Ge isotopic signatures, while the other group (mostly impact melt rocks) displays comparable or lower Ge isotopic signatures relative to the basement samples, but at higher Ge elemental concentrations.
Discussion and conclusions: These results imply at least 2 distinct processes affecting the rocks sampled in the drill cores: (1) heavy d74/70Ge isotopic signatures pos-sibly indicating impact induced volatilization, (2) signa-tures that would reflect secondary alteration. Moreover, data for Fe, Cu, Zn isotopes and HSE concentrations are currently collected for the same nineteen samples, in order to constrain the geochemical and isotopic signatures of the various lithologies of the Rochechouart impact structure. Along with geochemical and petrographic parameters, the nature of these syn- and post-impact processes, including melting and mixing of target rocks, volatilization, meteoritic contribution, and hydrothermal alteration, may be traced and refined.
References: [1] Rasmussen C. et al. (2020) Geochimica and Cosmochimica Acta 273, 313-330. [2] Lambert P. (1977) Earth and Planetary Science Letter 35, 258-268. [3] Koeberl C. et al. (2007) Earth and Planetary Science Letter 256, 534-546. [4] Lambert P. et al. (2016) Meteoritics & Planetary Science, A399. [5] Lambert P. et al. (2018) LPSC XXXIX, Abstract #1954. [6] Luais B. (2012) Chemical Geology 334, 295-311
